Signal modulators are utilized in a number of applications, for instance as part of the conversion from analogue signals to digital signals or vice versa. Time-encoding modulators are modulators that encode input signals into a time-encoded data stream.
One particular form of time-encoding is pulse-width modulation (PWM). In a PWM signal, an input value is encoded by the duration of a given output signal level, e.g. the duration or width of pulse of a first signal level, compared to the duration of any period(s) of any other signal level(s) in a cycle period. For a conventional two-level PWM signal, the input signal value may be encoded by the duty cycle of a pulse of a first signal level within the cycle period, i.e. the proportion of the cycle period spent at the first output signal level.
One particular application of a time-encoding modulator is as part of an analogue-to-digital converter (ADC) having a controlled oscillator, such as a voltage-controlled-oscillator (VCO).
In a VCO based ADC the input analogue signal may be used to control the VCO, which thus outputs an oscillation signal with a frequency dependent on the value of the input signal. The frequency of the oscillation signal can be determined, for instance by counting the number of cycles of the oscillation signal in a defined frame period, to provide a digital output indicative of the value of the input signal. The VCO can be implemented by a relatively simple ring oscillator, which is relatively low power, and thus VCO based ADCs can be relatively efficient in terms of power requirements and circuit area compared to alternative ADC architectures such as continuous-time or discrete-time sigma-delta ADCs.
However VCOs, especially those based on ring oscillators, typically have relatively poor linearity. Thus if the VCO is controlled directly by the analogue input signal the resulting digital output signal may have poor linearity. Whilst this may be acceptable for some applications, the poor linearity is disadvantageous for many audio applications.
A time-encoding modulator, for instance a PWM modulator, can be used to improve the linearity of a VCO based ADC by modulating the input analogue signal into a time-encoded signal which varies between two voltage states, where the value of the input signal is encoded by the relative durations of each state. The time-encoded signal is then used to control the VCO. As the VCO thus only operates with two input voltage levels the output is inherently linear. The average frequency of the oscillation signal in a frame period depends on the proportion of time spent at each voltage level, and thus depends on the value of the input signal.
Especially for audio applications however there is an increasing requirement for better still audio performance, e.g. for low levels of distortion such as THD, and other sources of non-linearity associated with operation of the VCO may impact distortion performance.